1. Field of the Invention
In general, the present invention relates to a projection-type picture display apparatus such as a projection television set and a screen used thereby. More particularly, the present invention relates to a projection-type picture display apparatus which has a small amount of inadvertent inclusion of external light, is capable of suppressing a reduction in contrast and capable of lowering the degree of deterioration of a picture quality by using an optical device having a structure comprising pixels laid out to form a matrix such as a liquid-crystal panel or a DMD (Digital Micromirror Device) as a picture generating source, and relates to a screen used by the projection-type picture display apparatus.
2. Description of the Related Art
With the picture source going diverse, the projection-type picture display apparatus is enjoying broad general popularity in the market by virtue of its marketability factors of a projection-type optical apparatus with a large screen such as a small weight, a low cost and a small size. On the other hand, in recent years, a projection-type picture display apparatus using a liquid-crystal panel as a picture generating source starts its participation in the market due to substantial improvement of the precision/fineness and the numerical aperture of the liquid-crystal panel. This projection-type picture display apparatus is designed into a configuration wherein a source picture displayed on the liquid-crystal panel is displayed as an enlarged picture on a screen in full colors by a projection lens unit.
In an optical system employed in this projection-type picture display apparatus, it is possible to adopt a three-panel technique employing three liquid-crystal panels as shown in FIG. 19 of Japanese Unexamined Patent Publication No. Hei 9-96759, or a single-panel technique employing only one liquid-crystal panel as shown in FIG. 1 of Japanese Unexamined Patent Publication No. Hei 4-60538. First of all, an optical system adopting a single-panel technique employing one liquid-crystal panel is explained by referring to FIG. 1.
FIG. 1 is a top-view diagram showing a partial cross section of a layout of a projection-type optical system adopting a single-panel technique employing one liquid-crystal panel.
As shown in the figure, a reflective mirror 29 directs a beam emitted by a white-color light source 28 implemented by a metal halide lamp, a canon lamp, a halogen lamp or a high-pressure mercury lamp to a converging lens 27 with a high degree of efficiency whereas a collimator lens 26 converts the beam into all but parallel white-color lights. Three dichroic mirrors 23, 24 and 25 with types different from each other are placed in front of the collimator lens 26. The dichroic mirrors 23, 24 and 25 exhibit characteristics to selectively reflect lights with the green, red and blue wavelengths respectively but to pass on other components. Symbols R, G and B in the figure denote respectively the red, green and blue lights split by the dichroic mirrors 24, 23 and 25. In this conventional configuration, the red-color light is taken as a reference while the blue-color and green-color lights are radiated to a liquid-crystal panel 22 from slanting directions relative to the red-color light.
The liquid-crystal panel 22 comprises pixels for the 3 primary colors, namely, red, green and blue. The pixels each exhibit an optical transmittivity representing the level of a luminance component of a picture signal. Thus, the red, green and blue lights are modulated in accordance with the level of the picture signal to create a desired image on the liquid-crystal panel 22. The image displayed on the liquid-crystal panel 22 is then projected by a projection-lens unit 21 on a screen 20 as an enlarged picture.
In order to radiate an image light emitted by the liquid-crystal panel 22 to the projection-lens unit 21 with a high degree of efficiency, an optical system including a convex lens for converging a light is typically provided between the liquid-crystal panel 22 and the projection-lens unit 21. It should be noted that such an optical system is not shown in FIG. 1.
The white-color light source 28 itself dissipates heat which can be a cause of a damage. On the other hand, the liquid-crystal panel 22 including a polarizing plate absorbs an incident light, dissipating heat which can also be a cause of a damage. In order to reduce an increase in temperature, a cooling fan not shown in the figure is used to forcibly cool the white-color light source 28 and the liquid-crystal panel 22 so that they can be used at a temperature in a desired range.
Next, the optical system adopting the conventional three-panel technique, that is, employing three liquid-crystal panels, is explained by referring to FIG. 2.
FIG. 2 is a top-view diagram showing a partial cross section of a layout of a projection-type optical system adopting the conventional three-panel technique employing three liquid-crystal panels. Configuration components shown in the figure identical with those of the optical system shown in FIG. 1 are denoted by the same reference numerals as those of the latter. The optical system shown in FIG. 2 has a configuration wherein a reflective mirror 29 collimates a beam emitted by a white-color light source 28 implemented by a metal halide lamp, a canon lamp, a halogen lamp or a high-pressure mercury lamp into all but parallel white-color lights. Two dichroic mirrors 31 and 32 with types different from each other are placed in front of the reflective lens 29. The dichroic mirrors 31 and 32 split the beam into color components which are then radiated to their respective liquid-crystal panels 33, 34 and 35. Pictures displayed on the liquid-crystal panels 33, 34 and 35 are synthesized by a color synthesizing prism 36 before being projected by a projection-lens unit 21 on a screen 20 as an enlarged picture. Since the operation of the projection-type optical system adopting the conventional three-panel technique employing the three liquid-crystal panels is the same as the conventional system shown in FIG. 1, it is not necessary to repeat its explanation.
In addition, a forced cooling system of the white-color light source 28 and the liquid-crystal panels each including a polarizing plate is the same as the optical system shown in FIG. 1. Furthermore, a radiation system to increase the efficiency of light utilization has become popular in recent years. The radiation system has a polarized-light synthesizing function for synthesizing P and S polarized lights generated by a polarization beam splitter as a result of splitting of a light emitted by the light source.
A rear-projection-type picture display apparatus employing the optical systems explained above is described by referring to FIGS. 3 and 4.
FIGS. 3 and 4 are each a side-view diagram showing a partial cross section of main components of the rear-projection-type picture display apparatus employing the projection-type optical system. In the figures, reference numerals 11 and 12 denote a radiation system including a light source and a projection lens respectively. Reference numeral 13 denotes an optical-path reflection mirror and reference numeral 14 denotes a screen. Reference numeral 15 denotes a case. The length of an optical path from the projection lens 12 to the screen 14 in the rear-projection-type picture display apparatus shown in FIG. 3 is equal to that of the apparatus shown in FIG. 4. Since the rear-projection-type picture display apparatus shown in FIG. 3 has a comparatively big depth, its height can be made relatively small. Since the rear-projection-type picture display apparatus shown in FIG. 4 has a comparatively big height, on the other hand, its depth can be made relatively small. In either of the rear-projection-type picture display apparatuses, by shortening the projection distance of the projection lens 12, that is, the length of the optical path from the projection lens 12 to the screen 14, a compact set can be implemented by employing only one optical-path reflection mirror 13. As the screen 14, a screen 40 having a two-sheet structure is normally employed. The screen 40 comprising a lenticular lens sheet and a Fresnel lens sheet is used in a projection-type picture display apparatus employing a Braun tube 43 shown in FIG. 10.
FIG. 10 is a side-view diagram showing a partial cross section of main components of the rear-projection-type picture display apparatus employing the projection-type Braun tube 43. It should be noted that, in the figure, reference numerals 40 and 41 denote a screen and a projection lens respectively whereas reference numerals 42 and 43 denote a bracket and a projection-type Braun tube respectively. Reference numeral 44 denotes an optical path of a light projected from the projection-type Braun tube 43 to the screen 40 provided at the end of a radiation path.
A detailed configuration of the screen 40 shown in FIG. 10 is shown in FIGS. 12 and 13.
FIGS. 12 and 13 are each a diagram showing a squint view of main components of the conventional screen 40. Components of FIG. 12 identical with those shown in FIG. 13 are denoted by the same reference numerals as the latter. The screen 40 shown in FIG. 12 comprises a Fresnel lens sheet 51a and a lenticular lens sheet 52. On the other hand, the screen 40 shown in FIG. 13 comprises a Fresnel lens sheet 51b and a lenticular lens sheet 52. The lenticular lens sheet 52 comprises a lenticular lens 54 on the incidence surface, a lenticular sheet 56 on the emission surface and a light absorbing layer 57 provided on a protrusion 55. The Fresnel sheet 51a has a flat incidence surface and a Fresnel lens 53 on the emission surface thereof. On the other hand, the Fresnel sheet 51b comprises a lenticular lens 58 provided on the incidence surface thereof and a Fresnel lens 53 provided on the emission surface thereof. As described above, the screen 40 shown in FIG. 12 is different from that shown in FIG. 13 in that, in the case of the former, no lenticular lens 58 is provided on the image-light incidence surface of the Fresnel sheet 51 while, in the case of the latter, the lenticular lens 58 is provided. It should be noted that the lenticular lens 58 has its longitudinal direction coinciding with the horizontal direction of the screen 40.
In a screen having a two-layer structure comprising a lenticular lens sheet 52 and a Fresnel lens sheet 51 used in a rear-projection-type picture display apparatus employing the conventional projection-type Braun tube, the horizontal-direction width of the screen of the light absorbing layer 57 provided on the picture-viewing side of the lenticular lens sheet 52 can be made larger than the width of the lenticular lens 56 on the emission surface. That is, if the width of the lenticular lens 56 on the emission surface is made small, the efficiency of the light utilization decreases. Thus, the width of the light absorbing layer 57 in the screen horizontal direction can not be made large. For this reason, there exists a phenomenon in which an external light is reflected to the lenticular lens 56 provided on the emission surface. This phenomenon raises a first problem that remains to be solved. The problem is that the inadvertent inclusion of an external light can not be reduced to a value below a predetermined amount and the degree of contrast deterioration can not be suppressed below a predetermined level.
The following description explains in detail reasons why the first problem arises.
FIG. 11 is a diagram showing a top view of a layout of the rear-projection-type picture display apparatus employing the conventional projection-type Braun tube on a horizontal plane in a simple and plain manner. As described above, the rear-projection-type picture display apparatus employs the conventional projection-type Braun tube in the horizontal-plane layout.
In FIG. 11, reference numerals 7R, 7G and 7B denote red, green and blue projection-type Braun tubes respectively whereas reference numerals 8R, 8G and 8B denote projection lens associated with the red, green and blue projection-type Braun tubes 7R, 7G and 7B respectively. Reference numerals 10R, 10G and 10B denote red, green and blue projected beams respectively. In the optical system of the rear-projection-type picture display apparatus, in actuality, there are reflective mirrors for reflecting the red, green and blue projected beams 10R, 10G and 10B. These reflective mirrors are omitted from FIG. 11. Reference numerals 13R, 13G and 13B denote optical axes of the projection lenses 8R, 8G and 8B respectively. The optical axes 13R, 13G and 13B cross each other, forming optical-axis convergence angles θ at a point S0 in close proximity to the screen 40.
As shown in FIG. 11, the projected beams 10R, 10G and 10B are each spreading before hitting the screen 40. Pay attention to a specific projected beam, that is, the red projected beam 10R for example. Lights of the red projected beam 10R emanating from the projection lens 8R toward pixels on the screen 40 are not parallel. Instead, those lights are becoming more distant from the main light hitting the center pixel on the screen 40 or the optical axis 13R.
A light hitting a pixel on the screen 40 in the same direction as the main light hitting the center pixel has the highest intensity among the lights reaching the pixels on the screen 40. That is, the direction of the main light is a direction of the highest intensity. Thus, to a viewer at a fixed position, only a portion of the picture is bright while surrounding portions are dark. This phenomenon is referred to as a color shift. In order to reduce the effect of the color shift, in the screen 40 shown in FIG. 12 or 13, a spread picture beam arriving at the incidence surface of the Fresnel lens sheet 51 is converted into an almost parallel beam passing through the Fresnel lens 53 and leaving for the lenticular lens sheet 52 for each of the red, green and blue colors.
FIGS. 14 and 15 are each a diagram showing a partial enlarged cross section of the lenticular lens cut in a direction A-A1 shown in FIG. 13.
In these figures, reference numeral 54 denotes the lenticular lens provided on the surface of incidence of the picture light and reference numeral 56 denotes the lenticular lens provided on the surface of emission of the picture light.
According to a conventional technology disclosed in Japanese Unexamined Patent Publication No. Sho 58-59436, the lenticular lens 54 provided on the incidence surface has a surface resembling a portion of the surface of an elliptical cylinder. The direction of the major axis of the ellipse coincides with the direction of a thickness between the incidence surface 54 and the emission surface 56 which is indicated by notation 1-1′ in FIGS. 14 and 15. One of the two foci of the ellipse is placed at an inner portion in close proximity to the lenticular lens 54 provided on the incidence surface. The other focus is placed at a position in close proximity to the lenticular lens 56 provided on the emission surface. The eccentricity e of the ellipse is set at a value approximately equal to the reciprocal of the refractive index n of the material of the lenticular lens sheet 52.
In such a configuration, all incident lights arriving at the lenticular lens 54 provided on the incidence surface in a direction parallel to the major axis of the ellipse are converged on the focus in close proximity to the lenticular lens 56 provided on the emission surface and spread in screen surface horizontal directions from this focus.
As for the lenticular lens 56 provided on the emission surface, the shape of the emission surface resembles the surface of an elliptical cylinder all but similar to the surface of an elliptical cylinder on the incidence-surface side. For an incident beam 60 at an edge shown in FIG. 14, the lenticular lens 56 provided on the emission surface functions to make the directional characteristic of the emitted light all but symmetrical with respect to the optical axis indicated by notation 1-1′ in the figure. Furthermore, for an incident beam 62 coming from a slanting direction of the red or blue color as shown in FIG. 15, the lenticular lens 56 provided on the emission surface has a correcting function to make the directional characteristic of the emitted light all but symmetrical with respect to the optical axis indicated by notation 1-1′ in the figure. In addition, the beam is not converged at a point due to frame aberration occurring at a location in close proximity to the emission surface at that time. Instead, the beam is spread in screen horizontal directions.
Inside the actual lenticular lens sheet 52, a scattering material 58 such as glass bead exists as shown in FIGS. 12 and 13, causing the beam to be actually further spread. Basically, however, the width of the light absorbing layer 57 in the screen horizontal direction can not be increased to a value greater than the width of the lenticular lens 56 provided on the emission surface. As a result, the quantity of light reflected due to inadvertent inclusion of an external light can not be reduced and a decrease in contrast can not be made lower than a predetermined value.
In addition, an incident beam 62 coming from the red or blue slanting direction is not converged at a point due to frame aberration occurring at a location in close proximity to the emission surface. Instead, the beam is spread in screen horizontal directions as described above. As a result, the width of the surface of the lenticular-lens surface can not be reduced. Thus, in the case of the lenticular lens sheet 52 adopting the conventional technique employing the lenticular lenses 54 and 56 on both the incidence and emission surfaces, there is raised a second problem that the lenticular-lens pitch in the screen horizontal direction can not be reduced to a value smaller than 0.5 mm.
For the reason described above, the total focus performance (horizontal resolution) of the rear-projection-type picture display apparatus is determined mainly by the lens pitch of the lenticular lens 52 which has a performance inferior to that exhibited by the projection-type Braun tube and the projection lens.
In addition, since the pitch of the lenticular lens 56 provided on the emission lens can not be reduced, a third problem is raised. To put it in detail, when an optical device having a structure comprising pixels laid out to form a matrix such as a liquid-crystal panel or a DMD (Digital Micromirror Device) is used as a picture generating source, the quality of a picture deteriorates due to moires generated on the entire screen 40. The generation of such moires is attributed to three causes, namely, projected and enlarged pixels on the screen 40, the lenticular lenses 54 and 56 provided on the lenticular lens sheet 52 and the Fresnel lens 53.